In recent years, consumer convenience as well as the development of new products has contributed to the increasing choice of processed vegetables and fruits available on the market. Ready-to-eat products, i.e. in a cut, washed and packaged form, may include lettuce (Lactuca sativa) and other leafy vegetables such as chicory (Cichorium intybus) and endive (Cichorium endivia), either individually processed or in mixed compositions. One of the most important and frequently encountered problems during harvesting, processing and storage of vegetables is the development of wound-induced surface discoloration visible by a pink discoloration at the wound surface of the plants or parts thereof which gradually turns brown after prolonged storage. Other crop plants such as potato (Solanum tuberosum), onion (Allium cepa), artichocke (Cynara cardunculus var. Scolymus), rice (Oryza sativa), corn (Zea mays), peach (Prunus persica), eggplant (Solanum melongen), celery and celeriac (Apium graveolens), apple (Malus domestica), banana (Musa acuminate), soy (Glycine max), pear (Pyrus×bretschneideri), wheat (Triticum aestivum), radish (Raphanus sativus), cabbage and cauliflower (Brassica oleracea) etc. may also be subject to the wound-induced surface discoloration visible on the plant or parts thereof, such as leaves, whole plant heads, fruits, inflorescences, seeds, curds, stems, tubers, bulbs and roots etc.
Wound-induced surface discoloration or wound-induced discoloration is caused by a strong wound response at and around the wound and leads to a rapid deterioration of the harvested and optionally processed product. Consumers consider discoloration of vegetables and fruits to be unattractive and to compromise the product quality, thus reducing the product's marketability and/or leading to a waste of harvested and optionally processed products.
The wound response is a means of a plant or part thereof to heal the wound and defend itself against pathogens by creating a new insulation barrier. The response is a complex biological response of a plant to physical injury such as cutting or bruising, and implies the activity of numerous proteins. The local response is mainly aimed at closing the wound surface which is effectuated by the local death of cells at or just behind the wound surface. In addition to these visible effects, other responses like increased respiration or ethylene production are known to be induced.
At the biochemical level, studies have shown that wounding can lead to the induction of the phenylpropanoid pathway (PP pathway) which is required for inter alia the production of polyphenols and other compounds important for the plant.
The first step of the PP pathway is the conversion of the amino acid phenylalanine into cinnamic acid by the phenylalanine ammonia-lyase (PAL). PAL is enhanced upon wounding by the induction of gene expression of at least one of its isoforms. This response leads to the formation of polyphenols which are oxidized by the polyphenol oxidase (PPO). PPO is residing in plastids and is released and activated upon wounding. Oxidation of polyphenols lead to the formation of highly reactive quinones, that can react with amino acids or proteins which leads to pink, brown or black discoloration.
In order to reduce the wound-induced surface discoloration in vegetables such as lettuce, many post-harvest and post-processing treatments have been developed and applied. Examples of chemical or physical treatments are the packaging of fresh cut leafy vegetables under a modified atmosphere, the application of edible coatings, heat-shock treatment and the addition of chemicals.
Although these treatments prevent the appearance of the wound-induced discoloration, the harvested and eventually processed product is still susceptible to discoloration if the package is damaged or shortly after opening the package. In addition, the use of chemicals and the need for specialized equipment for such treatments significantly increases costs. For these reasons, a more viable genetically-based solution which works to reduce wound-induced surface discoloration in plants is preferred.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.